446 DIRECTIVE FACTORS IN EVOLUTION: 



is the same. When Darwin says " Natural selection acts 

 by life and death . . . by the survival of the fittest and 

 by the destruction of the less well-fitted individuals ", he 

 describes lethal selection. Insects with reduced wings or 

 none at all abound in wind-swept islands like Madeira, the 

 flying insects having been blown out to sea and destroyed. 

 When Weismann points out that the animals best adapted 

 to the colour of their surroundings will secure the most 

 abundant food and multiply most prolifically, and will thus 

 increase the numerical proportion of others like themselves, 

 he is describing reproductive selection. If an advantageous 

 character is linked to an increase of fertility it will tend 

 to persist apart from lethal lopping off. In the cultivation 

 of a lawn one may eliminate the weeds by direct lethal 

 selection; but one may also stimulate the multiplication of 

 the grass by giving it a specific food which is not profitable 

 for the weeds. There is a special form of selection in the 

 sometimes fatal combats of rival males, and in preferential 

 mating when there is evidence of discrimination on the 

 female's part. There is social selection between rival ant- 

 hills, where community sometimes competes with commu- 

 nity, and, at the other pole, there may be selection between 

 potential egg-cells, the ovarian struggle sometimes ending 

 in the survival of one out of many, and selection between 

 the hundreds of sperm-cells in their race towards the ovum. 

 Allowing a wide margin for chance, the most vigorous and 

 perhaps the most sensitive spermatozoon will tend to succeed, 

 and the elimination of the others by the blocking of the 

 entrance to the egg will be for the advantage of the species. 

 As Weismann suggested, it is also possible that fluctuations 

 in the nutritive supply of the germ-cells, and inequalities 

 in the vigour and assimilating power of the hereditary con- 



